1. Field of the Invention
The present invention relates to an ink jet printer having a function of automatically correcting displacement of printing position occurring when an ink cartridge is mounted.
2. Description of the related Art
Conventionally, an ink jet printer having a function of automatically correcting displacement of printing position by forming a predetermined displacement detection pattern on a recording paper sheet by a recording head after an ink cartridge is mounted, reading the pattern by a displacement detection sensor and adjusting control timing of the recording head on the basis of the result has become commercially practical.
FIG. 6 shows a configuration of the displacement detection sensor for detecting the displacement of printing position used in the conventional ink jet printer. The displacement detection sensor 104 is comprised of a light emitting device 141 for emitting light on a recording paper sheet 2 at a predetermined radiation angle, a light receiving device 144 for receiving the light emitted from the light emitting device 141 and reflected by the recording paper sheet 2, a sensor housing 150 for accommodating these components and so on.
FIG. 7A shows a displacement detection pattern formed on the recording paper sheet 2 and a beam spot of light projected on the recording paper sheet 2 from the light emitting device 141. FIG. 7B shows an electric signal output from the light receiving device 144. Recently, in the ink jet printers, it is required to increase the detection accuracy of the displacement of printing position by the displacement detection sensor 104 following to further development of high resolution, so that the displacement detection pattern 80 shown in FIG. 7A comes in predisposed to be miniaturized. In order to detect such a miniaturized displacement detection pattern 80 with a high degree of accuracy, it is desirable to set the diameter of a beam spot 170 irradiated to the recording paper sheet 2 smaller corresponding to the miniaturization of the displacement detection pattern 80. However, in the configuration of the conventional displacement detection sensor 104 shown in FIG. 6, the diameter of the beam spot 170 depends on directional characteristics of the light emitting device 141 and a distance between the light emitting device 141 and the recording paper sheet 2, so that the designer of the ink jet printer cannot arbitrarily decide the diameter of the beam spot 170. As shown in FIG. 7A, the diameter of the beam spot 170 may become larger than a longitudinal dimension L and a width dimension W of the displacement detection pattern 80.
In such a case, as shown in FIG. 7B, an electric signal 190 having a waveform with small amplitude (small S/N) is output from the light receiving device 144. Thus, the detection error of the displacement detection sensor 104 becomes large and therefore, minute displacement of printing position cannot be corrected to respond to high-resolution printing satisfactory to the user.
Thus, there is the conventional ink jet printer, for example, having a configuration that a slit for detecting an edge of the displacement detection pattern is provided on a light receiving portion of the light receiving device so that the edge of the displacement detection pattern can be detected at the side of the light emitting device even when the diffused light is emitted from the light emitting device to ensure detection accuracy of the displacement detection sensor. However, such a light receiving device having the slit used in the printer contributes to an increase in manufacturing costs of the overall printer due to its high cost price.
Besides Japanese Laid-Open Patent Publication No. 2001-527472 discloses a displacement detection sensor in which a Fresnel lens provided at a side of a light emitting device and an aspherical lens provided at a side a light receiving device are formed in an integral manner. Furthermore, Japanese Laid-Open Patent Publication No. 2003-229995 discloses a displacement detection sensor having a plurality of light emitting devices for emitting lights of different colors and a lens assembly with a plurality of lenses for deflecting and focusing the light emitted from each of the light emitting devices at a predetermined position on a recording paper sheet. However, since a plurality of lenses must be integrally formed with a high degree of accuracy in both the displacement detection sensors, manufacturing costs become higher and thus, these sensors are not suited to printers for consumers requiring low price. Furthermore, since the Fresnel lens disclosed in Japanese Laid-Open Patent Publication No. 2001-527472 diffuses light in the direction other than the displacement detection pattern, projection efficiency of the light to the recording paper sheet is deteriorated, thereby decreasing the S/N of the signal output from the light receiving device.